I agree that it IS trespassing (not to mention downright rude!) to ride on anyone else's property without their express permission.
That said - it does not surprise me to hear that this neighbor of yours has "helped herself" to your land and your trails. I'll bet it didn't even OCCUR to her to check with you first - common sense and courtesy seems to be going the way of the dinosaur these days!
I would also be willing to bet, that if confronted, your neighbor will consider YOU to be the rude and disrespectful one...
Leasing a spoiled rotten trail horse...pretty - but a brat!